Infection with Borrelia burgdorferi leads to Lyme disease, which is the leading arthropod-borne infectious agent in the United States. Despite the prevalence of this disease, little is understood about the molecular pathogenesis of this infection. Inasmuch as bacterial adherence to host tissues is recognized as a critical component of the pathogenic process, we therefore propose to further characterize the B. burgdorferi decorin binding and BBK32 adhesins that bind to host decorin and fibronectin, respectively. Characterization of the adhesive properties of these borrelial microbial surface components that recognize extracellular matrix molecules will be conducted in concert with studies to address the infectivity of strains whose adhesin genes have been genetically inactivated either individually or in combination. To further characterize the borrelial lipoprotein adhesins and their respective mutants, we propose the following Specific Aims: (1) Define the role of the fibronectin binding BBK32 in B. burgdorferi pathogenesis;(2) Further characterize the role of the decorin binding proteins in B. burgdorferi pathogenesis;and (3) Evaluate the importance of selected lipoproteins in the molecular pathogenesis of B. burgdorferi infections. We have genetically inactivated the fibronectin binding adhesin in an infectious isolate of B. burgdorferi and have strong preliminary evidence that fibronectin binding participates in secondary colonization and/or protects the organism against clearance. This mutant represents the first genetic knockout in a lipoprotein encoding loci that has a known adhesive activity that targets a mammalian ligand. Similar analyses are planned for the genes encoding the decorin binding protein adhesins as well as genetic studies to evaluate a panel of novel lipoproteins for their importance for borrelial pathogenesis. Following initial characterization studies using in vitro correlates of infection, i.e., adhesion and invasion, we will then determine the molecular mechanism(s) these adhesins use to colonize, disseminate, and/or persist within the infected mammalian host(s).